Sanitization of granular activated carbon beds by applying chlorine dioxide.
When chlorinated water is treated in an equipment with granular activated carbon (GAC), the chlorine prevents it from containing microorganisms when entering. However, almost always, sooner or later, the water coming out of the equipment that has been chlorinated will show bacterial activity. In this methodology we explain the reasons for this phenomenon and the method of sanitizing the activated carbon using chlorine dioxide.
GAC used in water treatment is a very favorable medium for bacterial development. On the one hand, it traps organic molecules, many of which are biodegradable and can be food for these microorganisms. On the other hand, it has a rough surface that allows the bacteria a good fixation, which prevents hem from being treated. When the chlorinated water comes into contact with the top of the bed, the carbon reacts with the free chlorine and its disinfecting effect disappears. Since this reaction is faster than the adsorption process, no free chlorine reaches the bottom of the carbon bed, although organic molecules do.
When the water treatment process is stopped, the equipment depressurizes and a backflow can even occur (for example, by turning off the pump that is making the water flow from a cistern that is at a lower level). When this occurs, microorganisms can enter the interior. They can do so from outlets, sampling valves or from any leak. If these microorganisms get into the carbon, they fix themselves in its cracks and feed on the adsorbed organic matter.
When the process starts up again, the free chlorine does not affect them and they begin to reproduce and form a biomass that the water ends up dragging. The water starts to leave the equipment with a bacterial count. When the water coming out of the equipment goes through a UV or ozone disinfection stage, the subsequent bacteriological analyses will not show any bacterial activity, but the time will come when the water will have an anerobic smell; this is the typical smell of drainage.
That is why in a water treatment train for human consumption, bacteriological analysis of the effluent from the GAC equipment must be included in the quality control protocol. When these analyses do not comply with the standards of potability, it is necessary to sanitize the carbon bed.
What is the sanitization of activated carbon?
It consists of destroying the microorganisms found in it, as efficiently as possible. Even if they are completely eliminated, the bed will re-infect sooner or later. The absence of leaks, the pig tails at the sampling points, the remoteness of the treated water intakes from the equipment with CAG, well-closed valves and no leaks, as well as microbiological barriers (such as UV) decrease the frequency with the that will need to be sanitized.
Can charcoal from home water purifiers be sanitized?
In those where the application of any of the above methods is not practical (as in home water purifiers), the CAG impregnated with silver, which is bacteriostatic, is used. This is not recommended in industrial processes because it is more expensive than standard carbon, and silver is lost long before the CAG's capacity is depleted. Remember that the coal in a home purifier operates for a few minutes a day, while that of an industrial process operates for hours. Therefore, in the case of industrial processes, it is indicated to sanitize properly.
Effective sanitation of a GAC cannot be achieved by oxidants such as chlorine, ozone or hydrogen peroxide. GAC destroy them quickly and do not reach all points of the bed. Therefore, some of the microorganisms remain active.
A suitable compound for sanitizing GAC beds is chlorine dioxide.
6 steps of the procedure to sanitize the activated carbon with chlorine dioxide:
1. Calculate the volume of solution required.
This is necessary to ensure that the granular activated carbon (GAC) bed is completely flooded. GAC beds typically have a porosity of about 40%. That is, 40% of the bed´s volume is void space (Formed by the space between the carbon particles and the space inside the pores). The remaining 60% of the space is occupied by the solid. Example of calculation: the volume of the equipment that contains the GAC is 1800 l; half of it is occupied by a bed of GAC, and it is desired to flood the entire container with the chlorine dioxide solution. Calculate the volume of solution required:
Empty space in the GAC bed: 1800 L x 0.5 x 0.4 = 360 L
Empty space on the GAC bed: 1800 L x 0.5 = 900 L
Volume of solution required to flood the entire container: 1260 L
2. Preparation of the sanitizing solution.
In a separate tank, prepare the sanitizing solution with a concentration of 35 mg/L of chlorine dioxide. To do this, add 3 ml of commercial 10% chlorine dioxide solution to every 10 liters of water. Shake well to make the solution homogeneous.
3. Backwash the activated carbon bed.
In order to decompress the bed. It is important that expansion is achieved, so that the chlorine dioxide solution has access to the entire bed.
4. Flood the activated carbon with the sanitizing solution.
Make the sanitizing solution flow into the container to be sanitized, so that it is completely flooded with it.
5.- Let it rest for 8 hours.
As a minimum is the time we recommend to make effect on the bacteria that the bed of activated carbon, the maximum time is 24 hours.
6.- Backwash and rinse
Until a free chlorine test comes back negative.